Optical fiber sensors are often utilized to obtain various surface and downhole measurements, such as pressure, temperature, stress and strain. Examples of optical fiber sensors include optical fibers having a series of fiber Bragg gratings. The wavelength distribution from such gratings is affected by temperature and strain on the fiber, and thus such fibers can be used to measure temperature and strain, for example.
Some optical fiber sensors utilize cores doped with photosensitive materials. Photosensitive materials such as germanium are utilized to facilitate grating manufacture, but readily react with hydrogen at temperatures in excess of 100 C, which limits the performance in harsh environments such as those downhole. Furthermore, these materials are often heavily doped in the core to increase the numerical aperture as to improve the microbend resistance and macrobend resistance. An exemplary fiber includes a germanium doped silica core, a fluorine and phosphorous doped “matched cladding” and a pure silica outer cladding which is often the deposition tube in performs fabricated via Modified Chemical Vapor Deposition (MCVD).
Downhole environments are generally severe, and can expose fibers to conditions such as intense heat and pressure. When optical fibers such as germanium doped silica fibers are exposed to such an environment, attenuation losses can increase significantly. These losses are at least partly due to hydrogen losses in the fibers. For example, one type of loss, referred to herein as short wavelength edge (SWE)-induced loss, is associated with dopant (e.g., germanium) deficient-type defects along the fiber. Hydrogen atoms will bond to any open or weak bonds in the glass structure, such as to certain dopant atoms (e.g., Ge, Sn, Pb, Sb, B, P) in the vicinity of dopant-oxygen deficient centers, or to form SiOH and/or “dopant” OH. For germanium-doped fibers, for example, attenuation increases rapidly with increases in temperature.
An optical fiber with a pure silica core is often employed in downhole applications due to its inherent resistance to hydrogen induced attenuation at temperatures above 80 C. The pure silica core lacks the photosensitive materials preferred for standard grating fabrication processes at, for example, 248 nm or 193 nm.
Increases in photosensitive material concentration, such as germanium, increase the fiber sensors' sensitivity to hydrogen loss. Thus, high NA optical fibers having highly doped photosensitive cores, although having a relatively high NA and reduced sensitivity to micro and macrobends, exhibit high hydrogen induced loss, especially when exposed to downhole environments.